message/delivery-status

The machine-readable half of a bounce: when mail fails, the reporting server sends a multipart/report whose second part is this type — per-recipient fields like Action: failed, an enhanced status code, and the remote server's verbatim rejection, in the format RFC 3464 defines (2003, replacing RFC 1894). Anyone building on email infrastructure ends up parsing it, because bounce handling is how mailing lists prune dead addresses and how senders protect their reputation. The irony is well-aged: a format designed for machines is most often read by humans squinting at a "Mail delivery failed" message, while the humans it puzzles were never its audience.

Defined by RFC 3464 · IANA registration. Registry facts from the IANA media-types registry via mime-db.